A light source of the above type is known, for example, from DE 196 38 667 C2. The light source is described as a luminescence conversion LED. The LED (light-emitting diode) of the light source has, as active layer, for example, a semiconductor layer of gallium indium nitride (GaInN). This layer is electrically driven to emit primary radiation from a first wavelength region. The LED emits “blue” light. An intensity maximum of the primary radiation is at approximately 450 nm. The primary radiation is converted into secondary radiation with the aid of the phosphor. The phosphor is, for example, yttrium aluminum garnet activated or doped with cerium (YAG:Ce, Y3Al5O12:Ce).
The phosphor absorbs the primary radiation and emits secondary radiation from a second wavelength region. The phosphor emits “yellow” luminescent light with an intensity maximum that is dependent on the cerium concentration.
The phosphor is embedded, in the form of powder particles, in an epoxy resin or a low-melting inorganic glass. The epoxy resin or glass serves as a matrix for the powder particles. When the LED has been switched on, the phosphor of the powder particles is excited to emit the secondary radiation (luminescence). As soon as the LED has been switched off, no primary radiation is emitted, and consequently also no secondary radiation is emitted. The light source is extinguished. The light source is extinguished at almost exactly the same time as the LED is switched off.
For safety reasons, it may be desirable for the light source to continue to emit light for a longer period of time even in the event of a power failure.
Furthermore, it is known from DE-A 199 30 174 to operate LEDs by means of dimming with a defined duty factor. This technique is known as pulse width modulation (PWM). In general, however, only duty cycles of at best down to 1:100 are possible.